(1) Field of the Invention
The present invention concerns electric devices. More particularly, the present invention concerns electric devices such as capacitors and cables having as its components electric elements comprising insulating paper and synthetic resin films or synthetic resin films which have been impregnated with 1-phenyl-2-ethylphenylethane and/or 1-phenyl-2-isopropylphenylethane.
(2) Description of the Prior Art
Higherto, in electric devices such as transformers, cables and capacitors, mineral oil has been conventionally used as an insulating oil. However, because of high viscosity and high pour point of the mineral oil as well as its poor property to withstand higher voltages, the miniaturization of the above-mentioned electric devices has come to its limit. Accordingly, for the improvement of the property of withstanding high voltages of such electric devices, various insulating oils have been developed.
As synthetic insulating oils, several types of compounds have been proposed such as those of diarylalkane type, alkylnaphthalene type, etc. Prior art which discloses the diarylalkane type used also in the present invention is shown, for instance, in Japanese Patent Publication No. 1642/80. This prior art discloses as a diarylalkane type insulating oil, 1-phenyl-1-ethylphenylethane, 1-phenyl-1-isopropylphenylethane and 1-phenyl-1-xylylethane, which are of 1,1-diarylethane type. Among them, 1-phenyl-1-xylylethane (hereinafter referred to as 1,1-PXE) has recently been put into practical use. Although a 1,1-diarylethane-type compound, for instance, 1,1-PXE has an improved property of withstanding higher voltages as compared to mineral oil, its affinity to metals is not favorable. Incidentally, copper and lead-alloys such as solder are the metals indispensable to such impregrated electric devices. An insulating oil which has poor affinity to these metals shorten the life of the oil-impregnated electric device. Because in the case where such oil is brought into contact with the above-mentioned metals, the dielectric loss of the oil is raised with the result in increasing the dielectric loss of the oil-impregnated insulating element.
Moreover, since dielectric loss is directly proportional to the calorific value of a dielectric substance, the increase of dielectric tangent means the loss of electric energy as thermal energy. This phenomenon is not favorable also from the view point of economization of energy.
In recent years, plastic films have come to be largely utilized as dielectrics in order to reduce the dielectric loss of the electric device containing the dielectrics and to improve their properties of withstanding high voltage. Accordingly, with respect to the insulating oil for use in these electric devices, other than its properties of withstanding high voltages and its excellent affinity to metals, it is further required that the oil should not dissolve or swell such plastic film and it should have a low viscosity. The process of impregnation of such electric devices with oils is the easier as the viscosity of the oil is the lower and thus air void in such electric devices becomes less. The property of withstanding high voltage is enhanced because of the smaller amount of air void as well as the improved circulation of the oil between the layers of films in the electric devices in operation.